The current study investigated the relationship between duration of outlet obstruction, magnitude of bladder mass, and functional dysfunction on the rabbit urinary bladder. Following the production of obstruction with the "cuff model", bladder wet weight increased to twice control weight within one week, and then slowly to four times control weight by one month, and remained at this level for the six month period. Bladder capacity decreased significantly by one week but returned to control volumes by one month. The in vitro ability of the bladder to empty in response to field stimulation and bethanechol decreased significantly in the one and two week obstructed bladders and remained decreased for six months. One of the major observations of this study was the relatively large variation of bladder weight and histology observed for the one to six month obstructed rabbits. Although the bladders with mild mass increase (less than 3 gm./kg. body weight) had normal distribution of urothelium and muscular elements, the bladders with moderate mass increase (3 to 6 gm./kg.) had thick extrinsic connective tissue deposits and the bladders with severe mass increase (greater than 6 gm./kg.) had thick extrinsic and intrinsic connective tissue deposits and muscular degeneration. The percentage occurrence of mild, moderate and severe mass increase was approximately the same (58%, 30% and 12%, respectively) for the one, three, and six month groups. The bladders with mild mass increase had normal bladder capacities and increased pressure responses to field stimulation and bethanechol. The bladders with moderate-to-severe mass increase showed enlarged bladder capacities and had progressively smaller pressure responses. As the magnitude of bladder mass increased, the ability of the bladder to empty in response to field stimulation and bethanechol decreased proportionally. We conclude that the functional impairment of the bladder is related to the amount of extrinsic and intrinsic connective tissue and the degree of muscle degeneration.